The present invention relates to a projection display capable of projecting a color image to display an enlarged color picture.
A projection display for displaying an enlarged image of an image formed by a display device using reflection or transmission liquid crystal panel or micromirrors is required to form a sufficiently enlarged image on a screen and to have a small depth. A projection optical unit and a projection display intended to meet such requirements are disclosed in JP-A 2001-264627. The known projection optical unit and the known projection display project an enlarged image obliquely on a screen. Such an image projection mode will be referred to as “oblique image projection mode”. A method of optical adjustment of a projection display employing a curved mirror for oblique projection is disclosed in, for example, JP-A 2002-350774.
When an image is projected on a screen in an oblique projection mode at a predetermined angle to a normal to a major surface of the screen, for example, from a level below that of the screen, the image projected on the screen is subject to trapezoidal distortion and to optical aberration due to the difference in projection distance between light beams projected on an upper part of the screen and those projected on a lower part of the screen. A method of correcting such optical aberration disclosed in JP-A 2001-264627 corrects the trapezoidal distortion by a free curved mirror having a negative power and disposed between a projection optical system and the screen. The display device is translated greatly in a direction perpendicular to the optical axis relative to the coaxial projection optical system and the display device is tilted relative to a rotationally asymmetric projection optical system and is translated to correct the aberration.
When the aberration is corrected by such a method, an image displayed on the screen is likely to be longitudinally displaced. Therefore, a correcting mechanism for correcting the longitudinal displacement is necessary. A coaxial projection optical system required to have a large field angle needs many lenses and a large aperture. Any measures are not taken to cope with aberration and increase in distortion due to the deformation of optical parts caused by changes in ambient conditions.
Although an adjusting method that moves a free curved mirror is disclosed in JP-A 2002-350774, nothing is mentioned therein about aberration correction. Any measures are not taken to cope with increase in distortion due to the deformation of the free curved mirror caused by changes in ambient conditions
Thus the known technique uses separate means respectively for correcting trapezoidal distortion and aberration and hence needs many lenses each having a big diameter. The known technique has difficulty in achieving both satisfactorily reducing trapezoidal distortion and aberration when an image is projected in an oblique projection mode, and forming a display in a small depth and/or lowering the lower end of a screen, which will be referred to as “set compaction”. Any measures are not taken to cope with increase in aberration and distortion due to the deformation of optical parts caused by changes in ambient conditions. Both those known techniques do not mention about the shape of a free curved mirror that can be easily made.